Recently, there are large expectations for EL elements in the field of a flat-type display device. An EL element is characterized as to have a self-luminous property, a high level of visibility, a wide viewing angle, a fast response and the like. EL elements currently developed include an inorganic EL element using an inorganic material as a light emitting material and an organic EL element using an organic material as a light emitting material.
In an inorganic EL element in which an inorganic phosphor such as zinc sulfide is used as a light emitting material, electrons accelerated in a high electric field of 106V/cm collision-excites the light emitting center of the phosphor, and when they are relieved, light is emitted. Inorganic EL elements includes a dispersion EL element having such a structure that phosphor powder is dispersed in polymeric organic materials and electrodes are provided on top and bottom, and a thin-film EL element having two dielectric layers between a pair of electrodes and a thin film light emitting layer interposed between the dielectric layers. A dispersion EL element is easily manufactured, but the brightness is low and the service life is short, so it has been used only for limited purposes. On the other hand, as for a thin film EL element, an element having a dual insulating structure proposed by Inoguchi et al. in 1974 (see, for example, Japanese Patent Laid-open Publication No. 52-33491) shows high brightness and a long service life, which has been used practically for an in-car display or the like.
Here, a typical configuration of the thin film EL element will be explained by using FIG. 12. FIG. 12 is a sectional view vertical to a light emitting face of a thin film EL element 60. The EL element 60 has such a configuration that a transparent electrode 62, a first dielectric layer 63, a light emitting layer 64, a second dielectric layer 65, and a counter electrode 66 are laminated in this order on a transparent substrate 61. By applying alternating voltage between the transparent electrode 62 and the counter electrode 66, light is emitted from the light emitting layer 64. The first dielectric layer 63 and the second dielectric layer 65 interposing the light emitting layer 64 have a function of limiting current flowing through the light emitting layer 64, are capable of suppressing dielectric breakdown of the EL element 60, and act so as to obtain stable light emitting characteristics. Light emitted from the light emitting layer 64 is taken out from the transparent electrode 62 side.
Further, a display device may be configured by aligning a plurality of EL elements two-dimensionally in such a manner that the transparent electrode 62 is shared by a plurality of EL elements in the same row, and the counter electrode 65 is shared by a plurality of EL elements in the same row. In such a case, one transparent electrode extends in a row direction, and a plurality of transparent electrodes, parallel to each other, and a plurality of counter electrodes are stripe patterned so as to be made orthogonal to each other, and by applying a voltage to specific pixels selected by a matrix, it is possible to obtain a passive matrix driving type display device which performs any pattern display.
A basic driving method of a display device using the above-described EL element is realized such that transparent electrodes of the EL elements are used as data electrodes and counter electrodes are used as scan electrodes, and modulation voltage corresponding to display data determining whether to emit light or not is applied to the data electrodes while writing voltage is applied to the scan electrodes in a linear order. In this driving method, a superposed effect or an offset effect of writing voltage and modulation voltage is generated at parts where the scan electrodes and the data electrodes cross to each other (hereinafter referred to as pixels for short) in the EL element. The EL element has a voltage-brightness characteristic as shown in FIG. 13, and as an emission starting voltage Vth (hereinafter Vth indicates a positive real number), a high voltage as much as about 200V is required generally. In each pixel, when a voltage of not less than the emission starting voltage Vth is applied, light is emitted, and when a voltage of not more than the emission starting voltage Vth is applied, a state where light is not emitted is shown, whereby a desired display is obtained as a whole.
In a display device using such thin film EL elements, as a driving method for realizing gradation display of each pixel, there is known a voltage modulation method in which control is performed with voltage pulse amplitude applied to EL elements (see, for example, Japanese Patent Laid-open Publication No. 63-15590) or a pulse width modulation method in which control is performed with a pulse width (see, for example, Japanese Patent Laid-open Publication No. 01-307797). Among them, the voltage modulation method is capable of realizing a medium brightness by applying amplitude of modulation voltage applied to data electrodes in a multistage manner, but has a problem that gradation control accuracy is extremely low due to precipitous property and nonlinearity of voltage-brightness characteristics and hysteresis characteristics. On the other hand, the pulse width modulation method is capable of realizing a medium brightness by controlling the pulse width of modulation voltage applied to the data electrode in a multistage manner theoretically, but when driving pulse in a rectangle waveform is applied to the EL element, current contributing to light emission rises with a precipitous peak immediately after a rise of the voltage, and then shows a behavior of being attenuated rapidly as the same as charge current to the capacitor. The period in which the current flows is a short period of several μ seconds, and even though the pulse width is extended after the current is attenuated, current will not flow any more, so a brightness difference corresponding to the pulse width cannot be obtained. In order to obtain a sufficient gradation display by controlling the pulse width, it is required to control a multistage pulse width within the period of several μ seconds during which the current flows. However, if the pulse width varies slightly due to the response velocity of a driving circuit or the control accuracy of the pulse width or the like, the brightness largely changes. Therefore, gradation control by the pulse width modulation method is not suitable for EL elements.
Consideration will be given for the reason why the current contributing to light emission in an EL element rises with a precipitous peak immediately after a rise of voltage and then behaves to be attenuated rapidly as described above. One reason may be a fact that the EL element is a capacitive element. That is, the EL element has such a structure that the light emitting layer 64 is interposed between the dielectric layers 63 and 65, so it may be seen as a capacitive element from a viewpoint of equivalent circuit. In this case, when voltage pulse not less than an emission starting voltage is applied to the light emitting layer 64, the resistance of the light emitting layer 64 falls rapidly and electrons pass through the light emitting layer 64 in the high electric field to thereby excite the light emitting center, and then they reach the interface with the dielectric layer 65 and are retained. In this way, after light emitting operation is carried out, polarization charges remain in the light emitting layer. Hereinafter, this polarization charges are referred to as “first polarization charges” for short, and the potential difference caused inside the light emitting layer by the first polarization charges is referred to as a “first polarization voltage” for short. With the first polarization charges, a potential difference acting in a reverse direction to the outside voltage is caused inside the light emitting layer 64, whereby an effective voltage acted on the light emitting layer 64 becomes smaller than the emission starting voltage Vth due to an offset with the outside applied voltage, so current will not flow any more. Therefore, when voltage pulse is applied to an EL element, current contributing to light emission rises with a precipitous peak immediately after a rise of voltage, and then is attenuated rapidly.
This phenomenon will be explained in more detail by using FIG. 14, in which the horizontal axis shows the applied voltage V and the vertical axis shows the first polarization charges P. When voltage is not applied to an EL element and the first polarization charges do not exist in the light emitting layer, it is in a state of position A (polarization amount 0) in the Figure. Then, when a driving voltage Vr (voltage higher than emission starting voltage) in a pulse shape is applied so as to cause the EL element to emit light, it is transferred to a state of position B (polarization amount Pb) in the Figure as the applied voltage V rises, and then, even when the applied voltage V becomes 0, it is transferred not to a state of the first position A (polarization amount 0) but to a state of position C (polarization amount Pc). Namely, the first polarization charges remain in the light emitting layer even though voltage is not applied. This is considered as being based on a fact that when a voltage not less than the emission starting voltage is applied to the light emitting layer, electrons discharged from the proximity of the interface of one dielectric layer pass through the light emitting layer and reach the interface of the other dielectric layer, and are captured in a deep trap near the interface. Between the captured electrons and positive spatial charges in the light emitting layer, a steady electric field is formed and maintained. Then, when the polarity of the applied voltage between electrodes are reversed and a driving voltage −Vr in a pulse shape like the above-described one is applied, it is transferred from a state of position C (polarity amount Pc) via a state of position D (polarity amount 0) along the oblique line of negative voltage application to a state of position E (polarization amount Pe). Then, when the applied voltage becomes 0, it is transferred to a state of position F. In the state of position F, negative first polarization charges (polarization amount Pf) remain.
As described above, if a state where the first polarization charges remain in the light emitting layer continues, the first polarization voltage caused by the first polarization charges will be applied to the inside of the light emitting layer. Then, when light is emitted next time, the first polarization voltage is superposed with the outside applied voltage and applied to the light emitting layer. Therefore, although a voltage not more than the emission starting voltage Vth, providing operation of not emitting light, is applied, an effective voltage exceeding the emission starting voltage Vth is applied to the light emitting layer due to the first polarization voltage, which may cause false light emission.
Conventionally, in order to prevent such a false light emission at the time of next light emission, there has been proposed a method in which after a writing voltage is applied to each field, a polarization correction voltage of reversed polarization with respect to that of the writing voltage is applied so as to erase the first polarization charges (see, for example, Japanese Patent Laid-open Publication No. 03-69990). FIG. 15 is a time chart of a voltage applied to a light emitting layer of each pixel, showing an exemplary driving method for applying a polarization correction voltage. In a writing period 71, selective light emission is performed for each scan line, and then in a polarization erasing period 72, a polarization correction voltage of reverse polarity with respect to that of the writing voltage is applied. Further, C11 and C12 in FIG. 15 show pixels of different scan electrodes. In the Figure, an outside voltage applied to a pixel is shown by a continuous line, and the first polarization voltage caused by the remaining first polarization charges in the light emitting layer is shown by a broken line. In this conventional example, after a voltage not less than the emission starting voltage Vth is applied as a writing voltage in a linear order so as to perform light emission displaying, a polarization correction voltage near the emission starting voltage Vth is applied to all pixels. By applying the writing voltage, each pixel emits light, and then the first polarization voltage of reverse polarity with respect to that of the writing voltage is caused by the first polarization charges in the light emitting layer due to the first polarization charges remain in the light emitting layer. Then, when the polarization correction voltage is applied, a voltage in which the first polarization voltage and the polarization correction voltage are superposed is applied to the light emitting layer, and when the value of this voltage becomes the emission starting voltage Vth or more, the pixel emits light. After the emission, the first polarization voltage of reverse polarity with respect to that of the polarization correction voltage is applied to the light emitting layer, but the polarization correction voltage is so set as to be smaller than the first polarization voltage after the writing voltage is applied.
On the other hand, a conventional thin film EL element as shown in FIG. 12 does not have brightness sufficient for a high definition display such as a television, in general. Here, the relationship between an outside voltage applied to an EL element and a voltage allocated to a light emitting layer will be explained. Assuming that an outside voltage applied to an EL element is V′, the relative dielectric constant of a dielectric layer is εi, the film thickness is di, the relative dielectric constant of a light emitting layer is εp, and the film thickness is dp, the voltage V allocated to the light emitting layer is given by the following equation (1).V=εi·dp/(εi·dp+εp·di)·V′  (1)
As obvious from the equation (1), in order to allocate a voltage effectively to a light emitting layer, it is desirable to make it to be a thin film by using a material having a large relative dielectric constant of a dielectric layer. For improving brightness of an EL element, Japanese Patent Publication No. 07-44072, for example, proposes an El element in which an insulating ceramic substrate is used as a substrate and one dielectric layer constituting a dual insulating structure is a thick film dielectric layer. In the thick film dielectric layer, fine particles of a dielectric material having the perovskite structure such as BaTiO3, SrTiO3, PbTiO3, CaTiO3, Sr(Zr, Ti)O3, Pb(Zr, Ti)O3 are dispersed in an organic polymer matrix and made into paste, and then deposited by using a printing method, and then processed by heat treatment at high temperature whereby a large relative dielectric constant is realized. In general, a ferroelectric having the perovskite structure has large relative dielectric constant, which is preferable for making an EL element to have high brightness.